Bryson_MOLECULAR-CELL-D-22-00555_Figure_2C

A giant virus genome is densely packaged by stable nucleosomes within virions.
The two doublet histones of Marseillevirus are distantly related to the four eukaryotic core histones and wrap 121 basepairs of DNA to form remarkably similar nucleosomes. By permeabilizing Marseillevirus virions and performing genome-wide nuclease digestion, chemical cleavage and mass spectrometry assays, we find that the higher-order organization of Marseillevirus chromatin fundamentally differs from that of eukaryotes. Marseillevirus nucleosomes fully protect DNA within virions as closely abutted 121-bp DNA wrapped cores without linker DNA or phasing along genes. Likewise, we observed that nucleosomes reconstituted onto multi-copy tandem repeats of a nucleosome positioning sequence are tightly packed. Dense promiscuous packing of fully wrapped nucleosomes rather than “beads-on-a-string” with genic punctuation represents a new mode of DNA packaging by histones. We suggest that doublet histones have evolved for viral genome protection and may resemble an early stage of histone differentiation leading to the eukaryotic octameric nucleosome.
Variant assembly analyses of viral chromatin. Tapestation gel profiles of nucleosome assembly reactions containing Hz-He andHδ-Hγ tetramers and a 3-copy Widom 601 array with 40-bp linkers at histone:DNA ratios in lanes labeled: 0.25, 0.5, 1, 1.5, 2 and 2.5 after 5 min MNase treatment and DNA purification. Hz-He,Hδ-Hγ and Xenopus laevis (X.l.) assemblies are shown as controls.